I was driving on the highway the other day and my speedometer read 55 but I felt like I was doing more like 80. I slowed down to a reading of about 35 and kept pace with all the normal traffic. After unhooking the battery this fixed the problem, but a few weeks later I got off the interstate and the speedometer stuck at 70. Another day it wouldn't give me a reading at all. I can fix it by unhooking the battery but I'm sure there is some other underlying problem. Does anybody have any ideas on how to fix this, or how much it would cost to get it fixed?